One of the difficulties in applying robots on a widespread basis for assembly operations has been the lack of an effective sensor for sensing the magnitude of, and the location at which, tactile forces are applied to a part by a robot gripper. Sensing the magnitude of the applied force is crucial if crushing of the part by the gripper is to be avoided. Knowing the location at which the force is applied by the gripper aids in determining the shape and orientation of the gripped part, thereby facilitating assembly operations.
Current research efforts have led to the development of an advanced tactile sensor comprised of a plurality of "taxcels" or individual cells arranged in a matrix array for sensing tactile forces. An example of such a sensor is described by W. D. Hillis in his article entitled "High Resolution Imaging Touch Sensors" published in the International Journal of Robotics Research, Vol. 1, No. 2, Pages 33-34 (1982) incorporated by reference herein. The sensor described in the Hillis article comprises a printed circuit board having a plurality of parallel, spaced apart, electrically conductive members or foils thereon. A sheet of anisotropically conductive rubber, comprised of alternating electrically conductive and nonconductive strips arranged at right angles to the conductive foils, overlies the printed circuit board. A sheet of nylon mesh having openings therethrough is interposed between the circuit board and the rubber sheet to separate the conductive foils from the conductive rubber strips.
When force is applied against the rubber sheet, one or more of the conductive strips deforms through the perforations in the mesh and makes electrical contact with one or more of the conductive foils on the printed circuit board. The location of the crossing point or area of contact between the conductive strip and the conductive foil on the circuit board is indicative of the location at which the force is applied to the rubber sheet. The size of each area of contact varies in accordance with how great the applied force is and thus becomes a measure of the magnitude of the applied force.
In the Hillis sensor the openings through the mesh separating the rubber sheet from the circuit board are not uniform. The nonuniformity in the size and spacing of the perforations in the mesh introduces a certain randomness in the size and location of the areas of contact between the conductive strips and the conductive foils when the conductive strips are deformed in response to the application of force against the rubber sheet. As a consequence, the size and location of each area of contact no longer become an accurate measure of the magnitude, and location, respectively, of the applied force.
Also, in the Hillis sensor, the exposed surface of the rubber sheet (which is referred to as the sensory surface) undulates, that is to say, becomes wavy when the ends of the conductive strips are wrapped about the printed circuit board to make electrical contact with metallized regions on a second circuit board positioned therebeneath. The waviness in the sensory surface of the rubber sheet makes it difficult to obtain a uniform connection between the conductive strips and the conductive foils which may affect the area of contact therebetween, and hence the measurement of the magnitude of the applied force.
Accordingly, there is a need to provide a reliable technique for sensing tactile forces.